1. Field of the Invention
The present invention relates to an aqueous dispersion, composite powder and an organosol of a fluorine-containing polymer.
2. Description of the Related Art
Organosol
An organosol of polytetrafluoroethylene (hereinafter referred to as "PTFE") is known as a coating material which provides a coating film having good sliding and stainproofing properties and corrosion resistance. It is prepared by adding an organic medium having a boiling point higher 100.degree. C. to an aqueous dispersion comprising colloidal particles PTFE, which in turn is prepared by emulsion polymerization and then removing water from the mixture by heating (an azeotropic method. see Japanese Patent Publication No. 18775/1974).
Since the PTFE colloidal particles prepared by the emulsion polymerization are hardly redispersed once they are coagulated, a large amount of a surfactant is added to the aqueous dispersion to prevent coagulation.
To the same end, a copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (this copolymer being referred to as "FEP") which coagulates less than PTFE is used (cf. U.S. Pat. No. 2,937,156, GB Patent No. 1,094,349 and Japanese Patent Publication No. 17548/1973).
To prepare the organosol of PTFE, Japanese Patent Publication No. 17016/1974 discloses a phase transfer method which comprises adding PTFE to a phase transfer agent such as a water-soluble organic solvent or an aqueous solution of an electrolyte, and stirring the mixture to transfer PTFE particles to a phase transfer liquid such as an organic solvent which is insoluble or hardly soluble in water. In this method, the PTFE particles should be mixed to prevent coagulation of the particles.
The coating film prepared from the aqueous dispersion containing a large amount of surfactant tends to suffer from blisters or discoloration. The coating film prepared from the aqueous dispersion comprising FEP has inferior wear resistance so that its sliding property is greatly deteriorated during use.
In general, the colloidal particles of the high molecular weight PTFE having a molecular weight of, for example, about 1,000,000 or larger are easily fibrillated under small shear force, and the fibrils tangle together and cause coagulation. As such, it has been impossible to form an organosol from an aqueous dispersion of colloidal particles of PTFE having a high molecular weight by the phase transfer method or by redispersion of PTFE powder from the aqueous dispersion, which powder is often referred to as fine powder, in the organic solvent.
The formation of an organosol of PTFE particles may be made easy by decreasing of the molecular weight of PTFE to several thousand to several hundred thousand. This is done by using a large amount of a chain transfer agent during polymerization or by irradiation of the fine powder. However, when PTFE is used for lubrication, low molecular PTFE wears heavily.
Blending of PTFE with thermoplastic or thermosetting resins or elastomers
Due to the good properties of PTFE such as heat resistance, flame retardance, non-tackiness, low friction, chemical resistance, low dielectric characteristics and the like, PTFE is used to modify other thermoplastic or thermosetting resins or elastomers through blending. However, PTFE has little compatibility with other resins or elastomers, and PTFE particles are usually not sufficiently dispersed in a molded article of the blend. Thus, for a long time, it has been desired to increase the blending ability of PTFE without deteriorating the good performances of PTFE.
Low molecular weight PTFE which is a so-called "wax" is the most widely used for blending. The PTFE wax can be blended with other polymers to some extent, however, it has insufficient mechanical properties. An aqueous dispersion or powder of high molecular weight PTFE may be used for blending but has poor dispersibility in other polymers so that dispersion in a submicron order can hardly be expected.
To produce a milky molded article of a thermoplastic or thermosetting resin or the elastomer, an inorganic white pigment such as quartz fine powder, alumina hydrate, aluminum hydroxide and calcium carbonate is usually added and mixed in the base polymer. Since the inorganic pigment is harder than the base polymer, it often appears on the surface of molded article and deteriorates the surface smoothness when the pigment and the polymer are simply mixed. To keep the surface smoothness of the molded article, a special technique is required. When the amount of the inorganic pigment increases, the molded article itself becomes harder resulting in a decrease of impact resistance.
Since PTFE powder is white and heat resistant, it is preferred as an organic white pigment which is a substitute for in-organic white pigment. However, as described before, its addition and mixing is difficult.
As composite PTFE colloidal particles, Japanese Patent Kokai Publication No. 232109/1984 discloses composite particles each comprising a core of PTFE and a shell of a fluorine-containing elastomer, and Japanese Patent Kokai Publication No. 109846/1987 discloses composite particles each comprising a core of PTFE and a shell of a copolymer of TFE and chlorotrifluoroethylene (CTFE).
When the shell is made of the fluorine-containing elastomer, the composite particles have poor redispersibility in an aqueous medium or an organic solvent or poor ability to form the organosol. In particular, when blended with other polymers, the sliding property of the molded article is poor.
When the shell is made of the TFE/CTFE copolymer, the particles do not have good two-layer structures, and the molded article made from the blend of such composite particles and other polymer has poor wear resistance.